This disclosure relates to uplink (UL) power headroom (PH) reporting for carrier aggregation in wireless communications, in particular with reference to Long Term Evolution Advanced (LTE-A). Power headroom is the difference between a wireless transmit/receive unit's (WTRU's) maximum transmit power and the estimated power for a physical UL shared channel (PUSCH) transmission in the current subframe. A power headroom report (PHR) is an index reported by the WTRU to indicate the estimated PH. The WTRU sends the PHR to an evolved Node B (eNodeB or eNB), which may use the PHR to determine how much more UL bandwidth per subframe the WTRU is capable of using.
To support higher data rates and spectrum efficiency, the 3GPP long term evolution (LTE) system has been introduced into 3GPP Release 8 (R8). To further improve achievable throughput and coverage of LTE-based radio access systems, and to meet the International Mobile Telecommunications (IMT)-Advanced requirements of 1 Gbps and 500 Mbps in the downlink (DL) and UL directions respectively, LTE-Advanced (LTE-A) is currently under study in the 3GPP standardization body.
The LTE DL transmission scheme is based on an orthogonal frequency divisional multiple access (OFDMA) air interface. For the LTE UL direction, single-carrier (SC) transmission based on discrete Fourier transform (DFT)-spread OFDMA (DFT-S-OFDMA) is used. The use of single-carrier transmission in the UL is motivated by the lower peak to average power ratio (PAPR) or cubic metric (related to the non-linearity of a power amplifier) of the signal as compared to a multi-carrier transmission scheme such as OFDM.
For flexible deployment, LTE systems support scalable transmission bandwidths of either 1.4, 3, 5, 10, 15, or 20 MHz. The LTE system may operate in either frequency division duplex (FDD), time division duplex (TDD), or half-duplex FDD modes.
In an LTE system, each radio frame (10 ms) consists of ten equally sized sub-frames of 1 ms. Each sub-frame consists of two equally sized timeslots of 0.5 ms each. There may be either seven or six OFDM symbols per timeslot. Seven symbols are used with a normal cyclic prefix length, and six symbols per timeslot in an alternative system configuration may be used with an extended cyclic prefix length. The sub-carrier spacing for the LTE system is 15 kHz. An alternative reduced sub-carrier spacing mode using 7.5 kHz is also possible. A resource element (RE) corresponds to precisely one sub-carrier during one OFDM symbol interval. Twelve consecutive sub-carriers during a 0.5 ms timeslot constitute one resource block (RB). Therefore, with seven symbols per timeslot, each RB consists of 12×7=84 REs. A DL carrier may consist of a scalable number of resource blocks (RBs), ranging from a minimum of six RBs up to a maximum of 110 RBs. This corresponds to an overall scalable transmission bandwidth of roughly 1 MHz up to 20 MHz, but a set of common transmission bandwidths is usually specified, e.g., 1.4, 3, 5, 10, 15, or 20 MHz. The basic time-domain unit for dynamic scheduling in LTE is one sub-frame consisting of two consecutive timeslots. This is referred to as an RB pair. Certain sub-carriers on some OFDM symbols are allocated to carry pilot signals in the time-frequency grid. A given number of sub-carriers at the edges of the transmission bandwidth are not transmitted to comply with the spectral mask requirements.
In the DL direction, a WTRU may be allocated by an eNodeB to receive its data anywhere across the whole transmission bandwidth, e.g., an OFDMA scheme is used. The DL has an unused direct current (DC) offset sub-carrier in the center of the spectrum.
In the UL direction, LTE is based on DFT-S-OFDMA, or equivalently, SC-FDMA transmission. The purpose is to achieve a lower PAPR compared to the OFDMA transmission format. Conceptually, whereas in the LTE DL direction, a WTRU may receive its signal anywhere across the frequency domain in the whole LTE transmission bandwidth, a WTRU in the UL may transmit only on a limited contiguous set of assigned sub-carriers in an FDMA arrangement. This principle is called single carrier (SC)-FDMA. For example, if the overall OFDM signal or system bandwidth in the UL is composed of sub-carriers numbered 1 to 100, a first WTRU may be assigned to transmit its own signal on sub-carriers 1-12, a second WTRU may transmit on sub-carriers 13-24, and so on. An eNodeB receives a composite UL signal across the entire transmission bandwidth from one or more WTRUs at the same time, but each WTRU may only transmit into a subset of the available transmission bandwidth. In principle, DFT-S OFDM in the LTE UL may therefore be seen as a conventional form of OFDM transmission with the additional constraint that the time-frequency resource assigned to a WTRU consists of a set of frequency-consecutive sub-carriers. In the LTE UL, there is no DC sub-carrier (unlike the DL). Frequency hopping may be applied in one mode of operation to UL transmissions by a WTRU.
One improvement proposed for LTE-A is carrier aggregation and support for flexible bandwidth. One motivation for these changes is to allow DL and UL transmission bandwidths to exceed the 20 MHz maximum of R8 LTE, e.g., to allow a 40 MHz bandwidth. A second motivation is to allow for more flexible usage of the available paired spectrum. For example, whereas R8 LTE is limited to operate in symmetrical and paired FDD mode, e.g., DL and UL are both 10 MHz or 20 MHz in transmission bandwidth each, LTE-A may operate in asymmetric configurations, such as DL 10 MHz paired with UL 5 MHz. In addition, composite aggregate transmission bandwidths may also be possible with LTE-A, e.g., in the DL, a first 20 MHz carrier and a second 10 MHz carrier paired with an UL 20 MHz carrier and so on. The composite aggregate transmission bandwidths may not necessarily be contiguous in the frequency domain, e.g., the first 10 MHz component carrier in the above example may be spaced by 22.5 MHz in the DL band from the second 5 MHz DL component carrier. Alternatively, operation in contiguous aggregate transmission bandwidths may also be possible, e.g., a first DL component carrier of 20 MHz is aggregated with a contiguous 10 MHz DL component carrier and paired with a UL carrier of 20 MHz.
Examples of different configurations for LTE-A carrier aggregation and support for flexible bandwidth are illustrated in FIG. 1. FIG. 1a depicts three component carriers, two of which are contiguous and a third which is not contiguous. FIGS. 1b and 1c both depict three contiguous component carriers. There are two options for extending the LTE R8 transmission structure/format to incorporate the aggregated component carriers. One option is to apply the DFT precoder to the aggregate bandwidth, e.g., across all the component carriers in case the signal is contiguous, as shown in FIG. 1b and the right side of FIG. 1a. A second option is to apply the DFT precoder per component carrier only, as shown in FIG. 1c. It is noted that different carriers may have different modulation and coding sets (MCSs; i.e., a carrier-specific MCS), as shown in FIG. 1c. 
In the R8 LTE system UL direction, WTRUs transmit their data (and in some cases their control information) on the PUSCH. The PUSCH transmission is scheduled and controlled by the eNodeB using the UL scheduling grant, which is carried on physical DL control channel (PDCCH) format 0. As part of the UL scheduling grant, the WTRU receives control information including the modulation and coding set (MCS), transmit power control (TPC) command, UL resource allocation (i.e., the indices of allocated resource blocks), etc. The WTRU transmits its PUSCH on the allocated UL resources with the corresponding MCS at the transmit power controlled by the TPC command.
For scheduling UL WTRU transmissions, the scheduler at the eNodeB needs to select an appropriate transport format (i.e., MCS) for a certain resource allocation. For this, the scheduler needs to be able to estimate the UL link quality for the scheduled WTRU.
This requires that the eNodeB has knowledge of the WTRU's transmit power. In LTE, the estimated WTRU transmit power is calculated according to a formula where the eNodeB has knowledge of all components in the formula except for the WTRU's estimate of the DL pathloss. In LTE, a WTRU measures and reports back its DL pathless estimate to the eNodeB in the form of a PH measurement reporting quantity. This is similar to the concept of PH reporting in wideband code division multiple access (WCDMA) Release 6, where the PH is also reported for the eNodeB to perform appropriate UL scheduling.
In LTE, the PH reporting procedure is used to provide the serving eNodeB with information about the difference between the WTRU's transmit power and the maximum WTRU transmit power (for positive PH values). The information may also include the difference between the maximum WTRU transmit power and the calculated WTRU transmit power, according to the UL power control formula, when it exceeds the maximum WTRU transmit power (for negative PH values).
As explained above, in LTE, a single component carrier is used; therefore the definition of WTRU PH is based on one carrier. The WTRU transmit power PPUSCH for the PUSCH transmission in subframe i is defined by:PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)×PL+ΔTF(i)+f(i)}   Equation (1)
where PCMAX is the configured maximum allowed WTRU transmit power. PCMAX depends on the WTRU power class, allowed tolerances and adjustments, and a maximum allowed transmit power signaled to the WTRU by the eNodeB.
MPUSCH(i) the bandwidth of the PUSCH resource assignment expressed in the number of resource blocks valid for subframe i.
PO_PUSCH(j) the sum of a cell-specific nominal component PO_NOMINAL_PUSCH(j) and a WTRU specific-components PO_UE_PUSCH(j). PO_NOMINAL_PUSCH(j) is signaled from higher layers for j=0 and 1 in the range of [−126,24] dBm with 1 dB resolution and PO_UE_PUSCH(j) is configured by radio resource control (RRC) for j=0 and 1 in the range of [−8, 7] dB with 1 dB resolution. For PUSCH (re)transmissions corresponding to a configured scheduling grant, j=0 and for PUSCH (retransmissions corresponding to a received PDCCH with DCI format 0 associated with a new packet transmission, j=1. For PUSCH (re)transmissions corresponding to the random access response grant, j=2. PO_UE_PUSCH(2)=0 and PO_NOMINAL_PUSCH(2)=PO_PRE+ΔPREAMBLE_Msg3, where PO_PRE and ΔPREAMBLE_Msg3 are signaled from higher layers.
For j=0 or 1, αϵ{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1} is a three bit cell-specific parameter provided by higher layers. For j=2, α(j)=1.
PL is the DL pathloss estimate calculated by the WTRU.
ΔTF(i)=10 log10((2MPR/KS−1)×βoffsetPUSCH) for KS=1.25 and ΔTF(i)=0 for KS=0, where KS is a WTRU-specific parameter given by RRC.
  MPR  =            O      CQI              N      RE      for control data sent via the PUSCH without UL shared channel (UL-SCH) data, where OCQI is the number of CQI bits, including CRC bits, and NRE is the number of resource elements.
  MPR  =            ∑              r        =        0                    C        -        1              ⁢                  K        r                    N                  RE          ⁢                                                    for other cases, where C is the number of code blocks and Kr is the size for code block r. βoffsetPUSCH=βoffsetCQI for control data sent via the PUSCH without UL-SCH and βoffsetPUSCH=1 for other cases.
f(i)=δPUSCH(i−KPUSCH) if accumulation of TPC commands is not enabled based on the WTRU-specific parameter Accumulation-enabled provided by higher layers. δPUSCH is a WTRU-specific correction value, also referred to as a TPC command and is signaled to the WTRU in the PDCCH. KPUSCH is a subframe offset such that the value of f(i) in the current subframe i is the δPUSCH received KPUSCH frames before the current frame i. For FDD, KPUSCH=4 and for TDD, the value of KPUSCH varies.
The WTRU PH for subframe i is defined by:PH(i)=PCMAX−{10 log10(MPUSCH(i))+PO_PUSCH(j)+α×PL+ΔTF(i)+f(i)}  Equation (2)
The WTRU transmit power for the PUSCH in subframe i required by the UL scheduling grant (including radio bearer (RB) allocation, MCS, and power control command) without taking into account any maximum transmit power limitations, is denoted as PPUSCH_UG(i), and is defined asPPUSCH_UG(i)=10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)×PL+ΔTF(i)+f(i)   Equation (3)
Then, the actual WTRU transmit power on the PUSCH in Equation 1 may be rewritten as:PPUSCH(i)=min{PCMAX,PPUSCH_UG(i)}  Equation (4)
The PH formula for LTE in Equation 2 may be rewritten as:PH(i)=PCMAX−PPUSCH_UG(i)  Equation (5)
The existing definition of PH in LTE has been designed for the specific case of the SC-FDMA (or DFT-S OFDMA) air interface provided by R8 LTE. As such, it specifically applies to only one component carrier and only results in one single value measured and reported back by a WTRU for its entire UL direction and for a single multiple access scheme (one transmit antenna SC-FDMA). But this approach is not applicable to an LTE-A system using carrier aggregation, new multiple access schemes, MIMO schemes, or when operating in flexible bandwidth arrangements, where the eNodeB needs to know the PH information for multiple component carriers and/or multiple power amplifiers (PAs) to schedule and assign UL transmissions for the WTRU with the appropriate transmit power levels.
For example, suppose that three carriers are aggregated and used in an LTE-A system. The WTRU may have different maximum transmit powers on different carriers or have different pathloss values and/or open loop power control parameters leading to different transmit power levels on different carriers. At one sub-frame, the eNodeB may schedule the WTRU to transmit on two carriers (e.g., carriers 1 and 2). Given that the two carriers have different transmit powers, a single PH value would not be able to indicate the difference between the WTRU's maximum transmit power and the calculated transmit power (according to the power control formula) on each of the two carriers. Furthermore, when the eNodeB wants to schedule a future UL transmission on carrier 3, it will not know the PH information on carrier 3 (because the PH may not be reported, according to the concept in LTE). If carrier 3 is not contiguous to carriers 1 and 2, the DL pathloss on carrier 3 may not be derived reliably from the PH on carriers 1 and 2. The pathloss difference in non-contiguous carrier aggregation may be large, such as greater than 7 or 9 dB. This makes it difficult for the eNodeB to schedule UL transmissions with optimized power levels because the WTRU measured and reported PH value is not a representative metric equally valid for all the UL carriers assigned to that WTRU.
In addition to the existing reported PH values not being sufficient to accommodate multiple carriers, the signaling related to PH reporting is also insufficient. In an LTE system, transmission by the WTRU of a single value PHR for the entire cell bandwidth is triggered in one of the following ways: periodically (controlled by the PERIODIC_PHR_TIMER), if the pathloss has changed more than DL_PathlossChange dB since the last PHR and a predefined time has elapsed since the last report (controlled by the PROHIBIT_PHR_TIMER), or upon configuration and reconfiguration of a periodic PHR. Even if multiple events occur by the time a PHR may be transmitted, only one PHR is included in the MAC protocol data unit (PDU).
Methods and procedures are needed to estimate and report representative PH information when multiple carriers are assigned to a WTRU in an LTE-A system incorporating carrier aggregation. Furthermore, the transmission and signaling of the PH information also needs to be addressed to support efficient PH reporting in LTE-A.